Cutting tool insert

ABSTRACT

A polycrystalline diamond (PCD) compact and method for making the compact are provided. The method includes bringing a first PCD wafer and a second PCD wafer together at an interface in the presence of a bonding agent to form an unbonded assembly and bonding the wafers together at the interface at a pressure and temperature at which diamond is thermodynamically stable. The first PCD wafer is more thermally stable than the second PCD wafer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.15/017,992 filed Feb. 8, 2016, which is a Division of U.S. patentapplication Ser. No. 12/668,308 filed Mar. 26, 2010, now U.S. Pat. No.9,255,312 issued Feb. 9, 2016, which is a 35 U.S.C. § 371 ofPCT/IB2009/051479 filed on Apr. 8, 2009, published on Oct. 15, 2009under publication number WO 2009/125355 A and which claims the benefitof priority under 35 U.S.C. § 119 of South African Patent ApplicationNo. 2008/03078 filed Apr. 8, 2008, the disclosures of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

THIS invention relates to polycrystalline diamond compacts and moreparticularly to a method of manufacturing polycrystalline diamondcompacts.

A commonly used cutting tool insert for drill bits is one whichcomprises a layer of polycrystalline diamond (PCD) bonded to a cementedcarbide substrate. The layer of PCD presents a working face and acutting edge around a portion of the periphery of the working surface.Polycrystalline diamond comprises a mass of diamond particles containinga substantial amount of direct diamond-to-diamond bonding.

U.S. Pat. No. 4,224,380 discloses a compact consisting essentially ofself-bonded abrasive particles with an interconnected network of poresdispersed throughout. The compact is produced by bonding a mass ofabrasive particles into a self-bonded body through the use of asintering aid material under high pressures and temperatures (HP/HT).The body formed at HP/HT includes the self-bonded particles with thesintering aid material (e.g., cobalt or cobalt alloys) infiltratedthroughout the body. The infiltrant is then removed, for example, byimmersion of the body in an aqua regia bath. It has been discovered thatthe removal of substantially all of the infiltrant provides an abrasiveparticles compact which has substantially improved resistance to thermaldegradation at high temperatures.

U.S. Pat. No. 4,944,772 discloses a process for fabricating a supportedpolycrystalline diamond or CBN compact in general, though such processis especially adapted to the fabrication of a thermally-supportedpolycrystalline diamond or CBN compact. The process comprises forming asintered polycrystalline diamond or CBN compact having a surface andseparately forming a cemented carbide support having a support surface.The compact and support then are mated at their respective surfaces witha layer of diamond or CBN crystals having the largest dimension ofbetween about 30 and 500 micrometers interposed between said surfaces.Also, a source of diamond or CBN catalyst/sintering aid material isassociated with the layer of diamond or CBN crystals. The mated compactand support then are subjected to HP/HT conditions and for a timeadequate for converting said diamond or CBN crystals into apolycrystalline diamond or CBN compact and for producing a supportedpolycrystalline compact of at least two polycrystalline layers (i.e.bi-layer compact). Preferably, thermally-stable compacts are used in theprocess.

U.S. Pat. No. 5,127,923 discloses a highly consolidated abrasive compactwhich has enhanced particle-to-particle bonding, increased density andimproved thermal stability performance characteristics and which can bebonded directly to a supporting substrate. The compact is produced bysubjecting a mass of abrasive particles, e.g., diamond or cubic bornnitride, to multiple pressure cycles at high temperatures. Asolvent-catalyst sintering aid is employed in the initial pressurecycle. The compact then possesses residual interconnected porosity inthe particle mass which is filled with the solvent-catalyst. Dependingupon the degree of sintering, the solvent-catalyst can be removed byleaching or other suitable process. The removal of the solvent-catalystpermits further consolidation and sintering of the particle mass insubsequent pressure cycles. During the final pressure cycle, theabrasive mass can be bonded to a supporting substrate. In addition, anon-catalyst sintering aid, such as silicon, boron or metals renderednon-catalytic by the addition of silicon or boron which may form strongand chemically-resistant carbides, can be used in the second pressurecycle to enhance the sintering process and create a hard abrasivebonding matrix through out the particle mass.

Japan patent publication number JP 59-219500 discloses chemicaltreatment of a working surface of a PCD element. This treatmentdissolves and removes the catalyst/solvent matrix in an area immediatelyadjacent to the working surface. The invention is claimed to increasethe thermal resistance of the PCD material in the region where thematrix has been removed without compromising the strength of thesintered diamond.

U.S. Pat. Nos. 6,544,308 and 6,562,462 disclose a PCD element having abody with a working surface. A first volume of the body remote from theworking surface contains a catalyzing material, and a second volume ofthe body adjacent to the working surface is substantially free of thecatalyzing material.

There is a need for polycrystalline diamond compacts having excellentthermal stability in use combined with high strength and fractureresistance, and a cost-effective method for making them.

SUMMARY OF THE INVENTION

According to the invention there is provided a method for making apolycrystalline diamond (PCD) compact, the method including providing afirst PCD wafer; providing a second PCD wafer, the first PCD wafer beingmore thermally stable than the second PCD wafer, bringing the first andsecond PCD wafers together at an interface in the presence of a bondingagent to form an unbonded assembly; and bonding together the first andsecond PCD wafers at the interface at a pressure and temperature atwhich diamond is thermodynamically stable.

Both the first and second PCD wafers are in the form of pre-formedpolycrystalline diamond bodies, produced by methods known in the art.

The polycrystalline diamond compact which is produced has a layer orregion of PCD which is more thermally stable than a second layer orregion of PCD. The layers or regions are bonded along an interface.

The first and second PCD wafers have major surfaces on each of oppositesides thereof and the unbonded assembly is generally made by bringingtogether a major surface of one of the wafers and a major surface of theother wafer. The wafers are discrete and separate from each other priorto the bonding step.

Preferably the first PCD wafer is substantially free or devoid ofsolvent/catalyst for diamond. More preferably the first PCD element isat least partly porous.

The thickness of the first PCD wafer or of each first PCD wafer, ifthere is more than one, is preferably in the range from about 100 to 500micrometers.

Preferably the bonding agent comprises a solvent/catalyst for diamond.More preferably the bonding agent is disposed in interstices within thesecond PCD wafer. Alternatively or additionally, bonding agent may bedisposed on or proximate a surface of first PCD wafer a surface of thesecond PCD wafer, or surfaces of both the first and second PCD wafers.

The bonding agent must be present at the interface between the first andsecond PCD wafers at some stage during the step of bonding together thefirst and second PCD wafers.

The bonding agent may be provided in the form of a layer, and disposedintermediate the first and second PCD wafers. Such bonding agent maycomprise a solvent/catalyst for diamond or a refractory metal capable ofreacting with diamond to form a metal carbide, such as Mo, Nb, Ti, V,Cr, Zr, Hf, Ta or W.

The bonding step preferably results in direct diamond-to-diamond bondingbetween diamond in the first PCD wafer and diamond in the second PCDwafer.

When the first PCD wafer is porous, a thermally stable material ormaterial that does not readily react with diamond, or a precursormaterial capable of reacting with diamond to form such a material, maybe disposed proximate the upper surface of the first, at least porousPCD wafer within the unbonded assembly, with the purpose of melting andinfiltrating into pores within the first PCD wafer during the bondingstep. If, as is preferred, such material has lower melting point thansolvent/catalyst for diamond, it will infiltrate the first PCD waferbefore solvent/catalyst within the second PCD wafer melts and thushinder or prevent the solvent/catalyst from infiltrating into the firstPCD wafer. This preserves the thermal stability of the first PCDelement.

Preferably the ultra-high pressure is in the range from 3 GPa to 7 GPa,more preferably the ultra-high pressure is in the range from 3 GPa to 5GPa.

Preferably the temperature is at least 900 degrees centigrade, morepreferably the temperature is at least 1,000 degrees centigrade.

Where the bonding agent comprises solvent/catalyst for diamond, thetemperature is preferably such that the solvent/catalyst will dissolvediamond proximate or in the region of the interface and remainsubstantially solid, i.e. the solvent/catalyst will not substantiallyliquefy and infiltrate the first PCD wafer to any significant extent.

It is thus possible to use temperatures and pressures lower than wouldbe required to produce a layer of PCD from a mass of diamond particles.Substantial savings in the cost of manufacture can be achieved.

According to a further aspect of the invention the polycrystallinediamond compact comprises a polycrystalline diamond (PCD) table having aworking surface and a region of thermally stable polycrystalline diamond(TSPCD) adjacent the working surface.

In one preferred form of the invention, the PCD compact comprises a PCDtable and a region of TSPCD wherein the region comprises a relativelysmall region of the entire PCD. Thus, in one form of the invention thethickness of the first PCD wafer does not exceed 1200 μm. The firstlayer typically has a minimum dimension of approximately 100 μm. Thislayer may be further accompanied by an additional region of PCD which isnot thermally-stable i.e. it contains metallic catalyst/solvent phase.

The first PCD wafer will typically have a thickness and dimension suchthat in the abrasive element is the thermally stable region contributesno more than 60%, preferably less than 50% and most preferably less than40% to the overall height or thickness of the PCD table.

The first PCD wafer will typically be made of thermally stablepolycrystalline diamond which may be any known in the art. The thermallystable polycrystalline diamond will preferably be porous. The pores ofthe porous structure will generally be substantially empty, although thepores may contain a material which does not compromise the thermalstability of the layer. The thermally stable polycrystalline diamond maybe made by various methods known in the art. Typically, the method willinclude a HPHT sintering step, but other methods such as chemical vapourdeposition may be employed. The first PCD wafer, as produced, willtypically have a maximum dimension of 1.5 mm. In the case of a sinteringstep, it may include the use of a carbide substrate to providemechanical support and/or an infiltration source. The wafer is thenthinned, typically using mechanical means, to provide a maximumthickness of between approximately 1200 and 250 μm. The catalyst/solventbinder is then removed from the wafer using various known leachingtechnologies.

The second PCD wafer is typically made of polycrystalline diamondcomprising a bonding phase containing catalyst/solvent. The second PCDwafer may be made by methods known in the art. The catalyst/solvent willtypically be cobalt, iron or nickel or an alloy containing such a metal.

The diamond content of the PCD, whether first or second PCD wafer, ispreferably greater than 80 volume %.

The second PCD wafer may be bonded to a cemented carbide substrate.Alternatively, if the second PCD wafer is free standing, a body ofcemented carbide may be brought into contact with a surface of thesecond PCD wafer in the unbonded assembly. Bonding of the second PCDwafer to the cemented carbide substrate will occur during the bondingstep.

During the bonding step, the thermally stable nature of the first PCDwafer can be preserved by using lower temperatures at which thecatalyst/solvent remains essentially solid. Various other means such aspassivation, co-infiltration, or infiltration control can be used; or ifre-infiltration occurs, then the metallic infiltrant can be removed oraltered in a subsequent leaching or treatment step.

By “working surface” of the PCD cutting element is meant that surfacewhich is usefully employed in the operation of the cutter i.e. this willtypically include the top surface as well as the peripheral edgeportion, generally the top surface of the more thermally stable PCDregion.

In one form of this invention a plurality of discrete layers or wafersof PCD are bonded to one another, to form the resultant PCD table whichis bonded to a substrate, particularly a cemented carbide substrate. Aninfiltrant, which could be a conventional solvent/catalyst, may beincluded between some or all of respective layers or wafers, or thestack of wafers and the substrate, or a combination of these, to allowfor re-infiltration of an appropriate infiltrant during the synthesisprocess. In one version of this form of this invention, the discretelayers of PCD have generally the same composition, such that the PCDtable has generally the same composition as the individual layers orwafers. In an alternative form of the invention, the individual PCDlayers or wafers have different compositions to form, for instance, aPCD table with a composition gradient running through its thickness.

An advantage of stacking relatively thin porous, thermally stable PCDelements rather than providing a single relatively thicker one arisesfrom the fact that removal of solvent/catalyst binder from the PCDelement is an extremely difficult and time-consuming step. This step isnecessary since PCD elements typically include solvent/catalyst materialwithin interstices within the PCD structure as a consequence of themanufacture of sintered diamond bodies. The thicker the PCD element, themore time-consuming and costly is the step of solvent/catalyst removal,which step typically involves treating the element in an acid liquor forseveral weeks. In addition, the lower the average size of the diamondgrains within a PCD element, which may be required for improved wearresistance of the PCD element, the longer the treatment step takes. Themethod overcomes this problem by providing thinner PCD elements, whichrequire much less time to treat, and stacking them.

The invention provides, according to yet another aspect of theinvention, a polycrystalline diamond compact comprising a first layer ofpolycrystalline diamond bonded to a second layer of polycrystallinediamond, the first layer of polycrystalline diamond being more thermallystable and thinner than the second layer of polycrystalline diamond.

Preferably the thickness of the first layer of polycrystalline diamondis in the range 100 to 500 microns.

The first layer of polycrystalline diamond preferably comprisesthermally stable polycrystalline diamond, as described above.

The second layer of polycrystalline diamond is preferablypolycrystalline diamond containing a bonding phase comprising a diamondsolvent/catalyst, as described above.

Bonding between the two layers of the polycrystalline diamond compact ispreferably direct diamond-to-diamond bonding.

A PCD compact according to the invention and as produced by the methodof the invention is suitable for use in tools for cutting, machining,boring, drilling or degrading bodies comprising hard or abrasivematerials, such as rock, concrete, asphalt, ceramic, metal, compositesor wood. The PCD compact is particularly suited to applications in whicha working edge of the tool reaches elevated temperatures in use,particularly the drilling or boring of rock formations, as may becarried out in the oil and gas drilling industry. The PCD compact ispreferably bonded to a hard-metal substrate, preferably acobalt-cemented tungsten carbide substrate, the more thermally stablefirst PCD layer being disposed remotely from the substrate, with asecond, less thermally stable PCD layer being disposed intermediate thefirst PCD layer and the substrate. The first PCD layer thus provides arelatively thermally stable working surface and working edge forengaging the body or workpiece and improving the overall resilience ofthe compact against heating in use. The second PCD layer is preferablymore fracture resistant and stronger than the first PCD layer, and thusprovides robust support for it in use. Interstices within the second PCDlayer are preferably at least partly filled with a metal or metal alloy,more preferably a metal or metal alloy comprising a solvent/catalyst fordiamond.

An advantage of the method of the invention is that the properties ofthe first and second PCD wafers can be separately pre-determined, sincethey are both manufactured separately prior to being combined. Thismeans that they can be combined without substantial infiltration ofmaterial from one PCD wafer into the other. In particular, if the secondPCD wafer contains a solvent/catalyst for diamond, it would generally beundesirable for this material to infiltrate into pores within the firstPCD wafer, when such wafer is porous, since the presence ofsolvent/catalyst would substantially reduce its thermal stability. Thedegree to which solvent/catalyst would liquefy can be controlled bymeans of the temperature used during the bonding step. Preferably, thetemperature would be close enough to the melting point of thesolvent/catalyst material for it to have a solvent/catalyst functionlocally proximate the bonding interface, but not for substantial meltingto occur and consequently for molten solvent/catalyst material toinfiltrate into pores within the first PCD wafer. The temperatures usedfor the bonding step may therefore be substantially lower than thosethat are needed to sinter bulk PCD, a process that typically requiresmolten solvent/catalyst to infiltrate from a hard-metal substratecontaining solvent/catalyst material as a binder. Consequently, a lowerpressure could be used during the bonding step while maintaining acondition wherein diamond is thermodynamically stable, which isnecessary in order to avoid conversion of diamond into graphite in thepresence of a solvent/catalyst for diamond at a high temperature.

Another advantage of the invention is that the first PCD wafer may berelatively thin without risk of fracture during the bonding step. If thesecond PCD wafer was sintered during the bonding step rather than in aseparate sintering step, the first PCD wafer would need to be contactedwith an agglomerated mass of diamond particles during the application ofpressure, before the agglomerated mass had sintered to form a strong,inter-grown PCD support. This could result in the fracture of the firstPCD wafer during the bonding step. By preparing the second PCD waferprior to contacting it with the first PCD wafer, this problem is avoidedsince both the second PCD wafer functions as a rigid, stiff support forthe first PCD wafer during this step, which is especially important ifthe first PCD wafer is relatively thin.

A further advantage of the invention is that the pre-sintering of thefirst and second PCD wafers prior to the bonding step is believed toreduce the development of internal stresses within the PCD compact thatarise from combining the sintering and combining steps where theproperties, especially the thermal properties of the first and secondPCD wafers are substantially different.

A further advantage of the invention is that the filler material withininterstices within the second PCD wafer may be selected independentlyfrom the binder of substrate, since these components are pre-sinteredprior to the bonding step.

Yet a further advantage of the invention is that the first PCD wafer maybe treated independently from the substrate and the second PCD wafer torender it thermally stable. This treatment typically includes a step ofimmersing the element in acid for an extended period of time in order toleach out solvent/catalyst material from within interstices within it.If this step is carried out once the first PCD element is bonded to thesecond PCD wafer, which may be bonded to a hard-metal substrate, thelatter components need to be masked by some means to prevent them frombeing attacked by the acid. This masking process is not technicallytrivial and limits the types of leaching treatments which can beemployed without causing significant damage to the portions of thecutter which must be protected. By treating the first PCD wafer prior tobonding, this problem is avoided.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting preferred embodiments will now be described in more detail,by way of example only, with reference to the drawings FIGS. 1 and 2,which show schematic diagrams of cross sections of two embodiments ofunbonded assemblies.

DESCRIPTION OF PREFERRED EMBODIMENTS

In a preferred embodiment of the method described with reference to FIG.1, an unbonded assembly, 100, comprising a thermally stable first PCDelement (wafer in shape), 110, a less thermally stable second PCDelement (wafer in shape), 120, and a hard-metal substrate, 130, isprovided. The first PCD element is disposed remote from the substrate,and the second PCD element is disposed intermediate the first PCDelement and the substrate. The first PCD element is substantially freeor devoid of catalyst/solvent for diamond and the second PCD elementcontains a solvent/catalyst for diamond within internal interstices. ThePCD elements and the substrate are contacted and assembled into acapsule for use in an ultra-high pressure furnace, as is well known inthe art, and the assembly is subjected to a pressure and temperature atwhich diamond is thermodynamically stable. In a version of theembodiment in which the solvent/catalyst is cobalt, the pressure isabout 5.5 GPa and the temperature is about 1,400 degrees centigrade. Inanother version of the embodiment the pressure is 4.5 GPa and thetemperature is about 1,200 degrees centigrade.

In a preferred embodiment of the method described with reference to FIG.2, an unbonded assembly, 100, comprising more than one thermally stablePCD element, 110, each of which is referred to in this case as a firstPCD element, a less thermally stable second PCD element, 120, and ahard-metal substrate, 130, is provided. The first PCD elements aredisposed proximate each other and remote from the substrate, and thesecond PCD element is disposed intermediate the first PCD elements andthe substrate. The first PCD elements are substantially devoid ofcatalyst /solvent for diamond and the second PCD element contains asolvent/catalyst for diamond within internal interstices. The PCDelements and the substrate are contacted and assembled into a capsulefor use in an ultra-high pressure furnace, as is well known in the art,and the assembly is subjected to a pressure and temperature at whichdiamond is thermodynamically stable. In a version of the embodiment inwhich the solvent/catalyst is cobalt, the pressure is about 5.5 GPa andthe temperature is about 1,400 degrees centigrade.

The drawings do not show additional shims or sources of infiltrant whichmay be included in order to facilitate the bonding of the PCD elements.These may be inserted at interfaces between elements.

The PCD elements, 110 and 120, are produced using an ultra-high pressureand temperature sintering method, in which unbonded diamonds aresintered together at a pressure typically in the range from about 5 GPato about 8 GPa at a temperature typically in the range from about 1,300degrees centigrade to about 1,700 degrees centigrade in the presence ofa solvent/catalyst for diamond, or by means of chemical vapourdeposition (CVD). Both methods are well known in the art. The PCDelement may be sliced from a thicker PCD element by means ofelectro-discharge machining or a similar method. The element typicallyhas a diameter consistent with the final desired diameter of theabrasive element. The thickness of a first PCD element, 110, may bereduced if necessary by means of lapping or slicing (for example usingEDM), to provide a maximum thickness of approximately 1200 micrometers.This is the maximum thickness of PCD that is preferred for being subjectto treatment to remove substantially all solvent/catalyst containedwithin the element by means of leaching in acid. Various methods forremoving solvent/catalyst are known in the art, the most common beingimmersion of the PCD element into an acid bath for several days orweeks. Other known methods include electrolytic etching and evaporationtechniques.

In an embodiment in which a second PCD element is bonded to acobalt-cemented cemented carbide substrate, the portion of the secondPCD element adjacent the carbide substrate should have a grain size thatis less than 50 micrometers. Several PCD elements may be stacked suchthat their respective average diamond particle sizes are graded relativeto one another and to the uppermost first PCD element, this gradingbeing within the range from about 0.1 to 30 micrometers. Preferably, theintermediate layers have an average diamond grain size less than 30microns.

During the step of bonding together the PCD wafers and the substrate,solvent/catalyst material that may be present in the first layer or thesubstrate may re-infiltrate voids or pores in the first PCD wafer, whenporous. This can have a detrimental effect on the thermal stability ofthe working surface layer. Re-infiltration can be minimised if as low aspossible temperature is used while still achieving directdiamond-to-diamond bonding between the PCD wafers. There are severalother approaches to controlling or minimising this effect.

The first approach is to control the progress of the infiltrant front asit sweeps upwards into the wafer(s) region; such that it does notsignificantly contact the uppermost portions of the first PCD wafer orwafers. This can be achieved by control of the temperature and pressureover time during the bonding step, as would be appreciated by the personskilled in the art.

A second approach is partially to fill pores within in the first PCDwafer adjacent the working surface to a desired depth with a passivationcompound or material which effectively hinders or halts the infiltrantfront during the reattachment process.

A third approach is to co-infiltrate the porous first layer, typicallyfrom the top surface, with an alternative molten infiltrant materialduring the reattachment or bonding step. A material that has a lowermelting point than infiltrant sourced from the substrate is preferred inorder to fill the pores before the substrate infiltrant penetrates frombelow. However, it can be desirable to achieve simultaneous infiltrationfrom the top and bottom of the element or elements. For example, using asimilar process to that described in U.S. Pat. No. 5,127,923, the firstPCD layer or layers may be infiltrated with molten silicon or asilicon-based compound, resulting in the reactive formation of siliconcarbide within pores as the infiltrant comes into contact with thediamond network. Other molten infiltrants which are suitable includemetals such as aluminium, magnesium, lead and other similar metals oralloys containing these metals.

Example 1

A free-standing first PCD disc comprising bonded diamond grains having amultimodal size distribution and an average grain size of about 12micrometers was prepared by conventional means using ultra-high pressureand temperature and infiltrated cobalt as solvent/catalyst sinteringaid. The PCD disc was sintered in contact with a cobalt-cementedtungsten carbide substrate, which provided the source of cobalt forsintering the PCD and to which the PCD became integrally bonded duringthe sintering step. The substrate was removed by grinding it away,leaving a free-standing PCD disc. The disc was 17.4 millimetres indiameter and had a height of about 400 micrometers. The disc wasimmersed in a mixture of hydrofluoric and nitric acid for more than 96hours to remove substantially all of the cobalt from within intersticeswithin it, leaving the disc porous, i.e. a polycrystalline diamond withpores or voids within the polycrystalline structure.

A second PCD disc, having the same composition as the first disc, wasmanufactured in the same way as the first disc, but this time thesubstrate was not removed. The second PCD disc had a thickness of 1millimetre, and both the PCD and the substrate had a diameter of 17.4millimetres. The combined height of the PCD and substrate was 13millimetres.

The first, leached PCD disc was placed onto the top surface of thesecond PCD disc, and a silicon disc having diameter of 17.4 millimetreswas placed onto the upper surface of the first PCD disc to form anunbonded assembly. The unbonded assembly therefore comprised a first,thermally stable PCD disc remote from a substrate, with a second, muchless thermally stable PCD disc intermediate the first PCD disc and thesubstrate, and integrally bonded to the substrate, and a silicon disc ontop of the first PCD disc. The unbonded assembly was encapsulated withina jacket comprising a refractory metal cup, as is known in the art, andassembled into a capsule used for sintering PCD in a conventionalultra-high pressure apparatus. The purpose of the silicon was toinfiltrate into the upper porous PCD layer before the cobalt melted, andto react with the diamond to form silicon carbide, which is thermallystable. Once formed, the silicon carbide would prevent substantialinfiltration of cobalt from the second, intermediate PCD disc into thefirst, upper PCD disc, which it was intended should remain thermallystable. The assembly was subjected to an ultra-high pressure of about5.5 GPa and a temperature of about 1,400 degrees centigrade for aboutfive minutes to yield a PCD compact.

The PCD compact comprised an upper region of thermally stable PCD,comprising silicon carbide within internal interstices or pores of aninter-bonded network of sintered diamond grains bonded to a PCD regioncomprising cobalt within the interstices. The bonding at the interfacebetween these two PCD regions was in the form of directdiamond-to-diamond bonding between diamond in the first, upper layer andthat of the second, lower layer.

Example 2

A free-standing first, leached PCD disc and a second, unleached PCD discbonded to a substrate were prepared as in example 1.

A with niobium wafer having diameter of 17.4 millimetres was placed ononto the top surface of the second PCD disc, and the first, leached PCDdisc was placed onto the niobium wafer, in effect sandwiching theniobium wafer between the first and second PCD discs. A copper dischaving diameter of 17.4 millimetres was placed onto the upper surface ofthe first PCD disc to form an unbonded assembly. The unbonded assemblytherefore comprised a first, thermally stable PCD disc remote from asubstrate, with a second, much less thermally stable PCD discintermediate the first PCD disc and the substrate, and integrally bondedto the substrate, a niobium wafer intermediate the first and second PCDdiscs, and a copper disc on top of the first PCD disc. The unbondedassembly was encapsulated within a jacket comprising a refractory metalcup, as is known in the art, and assembled into a capsule used forsintering PCD in a conventional ultra-high pressure apparatus. Thepurpose of the copper was to infiltrate into the upper porous PCD layerbefore the cobalt melted, and thus to prevent substantial infiltrationof cobalt from the second, intermediate PCD disc into the first, upperPCD disc, which it was intended should remain thermally stable. Copperdoes not react readily with diamond and therefore does not compromisethe thermal stability of PCD.

The assembly was subjected to an ultra-high pressure of about 5.5 GPaand a temperature of about 1,200 degrees centigrade for about fiveminutes to yield a PCD compact. The temperature was selected to behigher than the melting point of copper, but lower than that of cobalt.

The PCD compact comprised an upper region of thermally stable PCD,comprising copper within internal interstices of an inter-bonded networkof sintered diamond grains bonded to a PCD region comprising cobaltwithin the interstices.

Example 3

A free-standing first, leached PCD disc and a second, unleached PCD discbonded to a substrate were prepared as in example 1.

The first, leached PCD disc was placed onto the top surface of thesecond PCD disc to form an unbonded assembly. The unbonded assembly wasencapsulated within a jacket comprising a refractory metal cup, as isknown in the art, and assembled into a capsule used for sintering PCD ina conventional ultra-high pressure furnace. The assembly was subjectedto an ultra-high pressure of about 5.5 GPa and a temperature of about1,250 degrees centigrade for about ten minutes to yield a PCD compact.The temperature was selected to be as close as practically possible tothe melting point of cobalt, without substantial cobalt meltingoccurring.

The PCD compact comprised an upper region of thermally stable,substantially porous PCD bonded to a lower PCD region comprising cobaltwithin the interstices. Direct diamond-to-diamond bonding betweendiamond in the first, upper layer and that of the second, lower layerwas evident, and the first PCD layer was substantially free of cobalt.

1. A polycrystalline diamond compact comprising a first layer ofpolycrystalline diamond bonded to a second layer polycrystallinediamond, the first layer of polycrystalline diamond being more thermallystable and thinner than the second layer of polycrystalline diamond. 2.A polycrystalline diamond compact according to claim 1 wherein the firstlayer of polycrystalline diamond is thermally stable polycrystallinediamond.
 3. A polycrystalline diamond compact according to claim 1wherein the second layer of polycrystalline diamond contains a bondingphase comprising a solvent/catalyst.
 4. A polycrystalline diamondcompact according to claim 1 wherein the bonding between the two layersis direct diamond-to-diamond bonding.